Campbell Scientific SDM-IO16 16-Channel Input/Output Expansion Module - Manual (Page 35)

Appendix A. General Principles of
Pulse and Frequency Measurements
A.1 Introduction
It is necessary to understand the general method of input measurements of the
SDM-IO16 to be able to easily comprehend the limits of frequency and duty
cycle resolution.
The microprocessor in the module runs an internal task that reads the status of
all 16 ports at a fixed frequency of 4096 Hz. Changes of state of each port from
one sample to the next are used to determine the start and end of pulses. This
sampling frequency determines the resolution and range of the pulse
measurements.
A.2 Frequency and Duty Cycle Measurement Range
To guarantee that a pulse is detected it must last longer than the time between
samples which is 244
μs. This sets the upper limit of signal frequency for
which pulses can be counted or frequencies measured. By implication, the
maximum frequency that can be measured is with a 50/50 duty cycle signal. If
the duty cycle is different from this, the maximum frequency measurable is
lower. This maximum frequency, measurable for a signal with a range of duty
cycles, can be expressed as the minimum of two functions:
f
max
= %min * 4096 / 100 (1)
f
max
= (100 - %max) * 4096 / 100 (2)
Where:
f
max
= maximum frequency at a specific duty cycle (Hz)
%min = minimum duty cycle in %
%max = maximum duty cycle in %
It also follows that for any given frequency (f) there will be a limit to the
maximum and minimum duty cycle that can be measured due to the restriction
of the minimum detectable pulse width. Using the same variables defined
above,
%min = f * 100 / 4096
%max = 100 - %min
It can be seen that the lower the frequency, the larger the measurable range of
duty cycle.
A-1